This invention relates to force-measuring apparatus such as a weighing scale and more particularly to a force-measuring apparatus compensated for creep.
The phenomenon known as creep encountered in a force-measuring device such as a load cell is exhibited as a change, either increase or decrease, in the output of the load cell with time under an applied load. When the load is removed the cumulative change in output, the creep error, remains. The output of the load cell then creeps toward zero as the creep effect of the load disappears. As different loads are applied for different durations and at different intervals the creep will vary accordingly.
The creep phenomenon is not well understood but is believed to have two main sources. The first source is changes in the grain structure of the counterforce. In theory, this source of creep results from mobile atoms in the crystalline grain structure of the load cell counterforce seeking positions of minimum energy in the lattice. Stress applied to the counterforce alters the energy pattern and causes the mobile atoms to seek new positions. Such migrations alter the lattice dimensions and change the strain detected by the strain gauges. When the stress is removed from the load cell the atoms migrate back toward their original positions and the output of the load cell creeps back to its original, or zero balance, reading. The error in the load indication introduced by the creep phenomenon is a function of both the magnitude of the load and the time elapsed from its application or removal.
A second source of creep is the relaxing of the adhesive bond between the counterforce and the strain gage and/or between the grids of the gage and the gage backing. Often the two aforementioned sources (counterforce grain structure and adhesive bond) tend to cancel each other. Strain gages usually are available with differing geometrical configurations which compensate more or less for the creep in the counterforce to which the gages will be attached. Thus, gages are matched to the counterforce to minimize creep. Although such methods greatly reduce the effects of the inherent creep on the load cell output further corrections are still needed for high accuracy applications.
Until recently, little effort was made to electronically compensate for the effects of creep on weighing accuracy. In U.S. Pat. No. 4,412,298 issued Oct. 25, 1983 to Feinland et al, a method is suggested for compensating for creep in a digital weighing scale. In that patent, a tare weight is stored prior to application of a load. After a load is applied and scale motion has ceased, weight readings are taken at intervals. If the cumulative difference in readings exceeds a predetermined amount the difference is added to the stored tare weight so that a true tare weight is maintained after removal of the load.